The invention relates to a process for the production of nodular cast iron with a large number of graphite nodules. The invention also relates to a casting produced using this process.
The production of castings, compared to welding, machining or deforming metal, has the considerable advantage that a product can be formed in one go and then scarcely requires any further treatment. The designer of a product also has considerable design freedom when determining the shape of the casting, and the castings can be produced in large numbers at relatively low cost. However, it is a drawback that most metals, such as aluminium and steel, shrink considerably during solidification, with the result that internal shrinkage cavities are formed and it is difficult or impossible to prevent porosity.
Cast iron behaves differently, since during solidification the carbon in the molten material is precipitated in the form of graphite particles. This formation of graphite goes hand-in-hand with an increase in volume, so that it is possible to compensate for the shrinkage of the iron. As a result, cast iron can in principle be free of shrinkage cavities and porosity.
With nodular cast iron, graphite particles which are more or less spheroidal are formed, so that they cause less of a notch effect in the cast iron. Consequently, nodular cast iron has mechanical properties which are comparable to those of steel.
Although the mechanism of nodule formation in nodular cast iron is not yet fully known, in practice a number of standard treatment techniques have been developed and patented. The starting point is a cast iron with a basic composition, the so-called base iron, containing, for example, 3.5% C, 2% Si,  less than 0.02% S, and other standard alloying elements which have a controllable influence on the graphite structure. During the preliminary treatment, which is usually carried out in a treatment ladle or casting ladle, magnesium is usually added to the molten material n order to achieve a freely dissolved magnesium content of 0.015 to 0.06% Mgxc2x10.005%. Often, small amounts of cerium, calcium and any other alkaline metal and alkaline-earth metal elements are also added. This preliminary treatment is known as nodulization or Mg treatment. After this nodulization, an inoculant is added to the cast iron, so that inoculation nuclei are formed in the molten material, around which inoculation nuclei the carbon can crystallize out in the form of graphite. This treatment is known as inoculation. Various compositions are in use as inoculant. The inoculant is preferably only added to the casting stream at the last moment, for example in the form of grains which just have time to dissolve in the molten material. It has been found that earlier addition of inoculant leads to a lower number of nodules per mm2 in the nodular cast iron. To carry out the nodulization and inoculation in one treatment following the casting process, it is possible to use a device in which the reactions generally take place under an inert protective gas.
A process of this type is described in French Patent 2511044. According to this document, an inoculant bearing the tradename xe2x80x9cSphxc3xa9rixxe2x80x9d is used, comprising a ferrosilicon alloy with 70-75% silicon, containing 0.005% to 3% of at least one of the metalloids bismuth, lead or antimony, and 0.005% to 3% of at least one metal from the group of rare earths. (All percentages in this text are given as percent by weight).
It is generally known that in practice it is very difficult to use conventional casting techniques to produce castings with a wall thickness of less than 5 mm which are free of primary carbides if unheated sand moulds and gravity die-casting are used. with a wall thickness of less than 5 mm, the cooling rate during solidification in the sand mould into which the cast iron is poured is so high that, in an optimum nucleation state according to the methods known hitherto, there are insufficient nuclei for complete graphitization to preclude the lowest form of white solidification. The excessively long diffusion distances to the graphite nuclei which are present will cause some of the dissolved carbon to form primary carbides or cementite in accordance with the metastable Fexe2x80x94C system instead of nodular graphite according to the stable Fexe2x80x94C system.
It is an object of the invention to provide an improved process for the production of nodular cast iron.
It is another object to provide a process for producing thin nodular cast iron which is free of cementite without using a heat treatment specifically for this purpose.
It is yet another object to provide a process which prevents the formation of undesirable primary carbides in thin walls.
It is yet another object of the invention to provide a process with which a microstructure of nodular cast iron is obtained in relatively thin wall thicknesses.
It is another object of the invention to provide a relatively simple process with which castings made from nodular cast iron can be produced with thinner wall thicknesses than has hitherto been possible.
It is yet another object of the invention to provide a process with which thin walls of castings can be produced from nodular cast iron with a number of graphite nodules which is higher than customary.
It is yet another object of the invention to provide a process with which thin-walled castings can be produced from nodular cast iron with larger dimensions than has hitherto been possible.
Typically the casting, in a wall with a thickness of between 2 and 5 mm, has a predominantly ferritic steel matrix.
It is also an object of the invention to provide castings made from nodular cast iron in which the above objectives are achieved.
According to a first aspect of the invention, one or more of the above objects are achieved with a process for producing nodular cast iron with a high number of graphite nodules, comprising the following steps:
preparing molten base iron for casting castings of nodular cast iron;
adding Mg to the molten base iron;
using a casting stream to cast the cast iron into a casting mould, an inoculant being added to the casting stream,
characterized in that between the addition of the Mg and the addition of the inoculant to the casting stream, a preliminary inoculation using a further inoculant is carried out as an additional step.
Surprisingly, it has been found that adding a further inoculant during an additional step has a very favourable effect on the number of graphite nodules formed. This preliminary inoculation with the further inoculant is all the more surprising since hitherto it has always been observed that the casting-stream inoculant should be added as late as possible in the process in order to form as many inoculation nuclei in the molten material as possible. When the inoculant was added earlier, it was observed that the effect of adding the inoculant decreased. Therefore, hitherto the inoculant has only been added to the casting stream which is used to fill the casting moulds. This addition takes place in an accurately metered manner.
With the process according to the invention, in which the further inoculant is added as an additional step, it is possible to produce castings from nodular cast iron in a conventional way without an additional heat treatment being required, while the castings can have walls with a wall thickness which is less than the previously customary minimum wall thickness of 5 mm. It has proven possible, with the aid of the process according to the invention, to produce castings from nodular cast iron with walls with a wall thickness of between 2 mm and 5 mm without white cast iron being formed. The process according to the invention is therefore eminently suitable for the production of components for the automotive industry which are subjected to relatively heavy loads and have hitherto been produced by, for example, welding from steel sheet.
Preferably, the preliminary inoculation with the further inoculant is carried out at most approximately 30 minutes before casting, preferably at most 15 minutes before casting. The preliminary inoculation can then be carried out well before the actual casting process, without the time at which the preliminary inoculation is to take place being critical.
According to one embodiment of the process, the Mg is added in a treatment or casting ladle and the further inoculant is added to the treatment or casting ladle packaged in a wire component. In this embodiment of the process, the treatment ladle also serves as casting ladle for casting the cast iron into the casting mould. The preliminary inoculation with the further inoculant in the form of a wire component is carried out independently and after the Mg treatment has beer completed.
According to another embodiment of the process, the Mg is added in a treatment ladle and the further inoculant is added to a casting stream leading from the treatment ladle into a casting ladle. In this embodiment of the process, the cast iron is firstly poured from the treatment ladle into a casting ladle. During this step, the further inoculant is added, so that the preliminary inoculation with the further inoculant is therefore carried out independently of the Mg treatment and is also spatially separate therefrom.
Advantageously, the further inoculant is identical to the casting-stream inoculant. It is then possible to make do with one type of inoculant, so that there can be no confusion as to which inoculant is to be used when.
The first inoculant preferably consists of a FeSi alloy containing approximately 70% Si and approximately 0.4% Ce misch-metal, 0.7% Ca, 1.0% Al and 0.8% Bi, and inevitable trace elements.
According to a preferred process, approximately 0.3% of the further inoculant is added during the additional step, the further inoculant having the same composition as the casting-stream inoculant. This quantity of the further inoculant with the abovementioned composition is sufficient to form a sufficiently high number of inoculation nuclei, obviously in conjunction with the use of the casting-stream inoculant.
Preferably, the amount of C in the base iron is made to be greater than or equal to 3.7% and the amount of Si is made to be as high as possible, so that it is possible to cast thin-walled castings. This composition of the molten material, in conjunction with the inoculants, has a beneficial effect on the number of graphite nodules formed.
For castings with a wall thickness of approximately 2 mm, it is preferable to use base iron containing approximately 4.0% C, and for castings with a wall thickness of approximately 3 mm it is preferable to use base iron containing approximately 3.8% C.
The Mg is preferably added as pure Mg or as a prealloy, such as NiMg15 or FeSiMg.
According to a preferred process, after the addition of Mg the amount of free Mg in the molten base iron is equal to approximately 0.020% for castings which are to be cast with a wall thickness of approximately 2 mm, is approximately 0.025% for castings with a wall thickness of approximately 3 mm, and is approximately 0.030% for a wall thickness of approximately 4 mm.
Preferably, a greater amount of casting-stream inoculant is added as the desired wall thickness of the casting to be cast becomes thinner. The addition of more casting-stream inoculant results in more inoculation nuclei being formed in the molten material and therefore more graphite nodules being formed in the casting. A greater number of graphite nodules is desired as the wall becomes thinner.
A second aspect of the invention provides a casting made from nodular cast iron which according to the invention has a wall with a wall thickness of less than approximately 5 mm, in particular 2 to 4 mm, by using the process described above. Castings of this type made from nodular cast iron which have at least one wall with a wall thickness of less than 5 mm are for many application areas, such as the automotive industry, a good substitute for traditionally formed components, such as heavy nodular cast iron, forgeable steel, cast steel or a welding composition, or for non-traditionally formed components, such as a heat-treated Al casting, since they can be produced at lower cost in greater numbers and are also lighter in weight, while also satisfying the functional requirements, in particular with regard to the strength.
The number of graphite nodules per mm2 in the casting preferably increases as the wall thickness becomes smaller, preferably being approximately 2000 nodules per mm2 for a wall thickness of approximately 3 mm and preferably being approximately 6000 nodules per mm2 for a wall thickness of approximately 2 mm. A number of nodules of this level is desirable in order to prevent white solidification of the cast iron at such thicknesses.
The casting preferably has dimensions which are at most 300 by 300 by 400 mm. These dimensions are large enough for most applications in which thin-walled castings can be used.